1. Technical Field
This invention relates to an integrated pressure sensor with double measuring scale and a high full-scale value. In particular, the following description makes specific reference, without this implying any loss of generality, to the use of this pressure sensor in a BBW (Brake-By-Wire) electromechanical braking system.
2. Description of the Related Art
As is known, traditional disc braking systems for vehicles include a disc that is fixed to a respective wheel of the vehicle, a caliper associated with the disc and a hydraulic control circuit. Pads (normally two in number) of a friction material and one or more pistons connected to the hydraulic control circuit are housed inside the caliper. Following the operation of the brake pedal by a user, a pump in the hydraulic control circuit pressurizes a fluid contained within the circuit itself. Accordingly, the pistons, equipped with sealing elements, leave their respective seats and press the pads against the surface of the disc, thereby exerting a braking action on the wheel.
Recently, so-called “Drive-by-Wire” systems have been proposed, which provide for the electronic control of a vehicle's main functions, such as the steering system, the clutch and the braking system, for example. In particular, electronically controlled braking systems have been proposed that envisage the substitution of hydraulic calipers with electromechanical actuators. In detail, suitable sensors detect the operation of the brake pedal and generate corresponding electrical signals that are then received and interpreted by an electronic control unit. The electronic control unit then controls the operation of the electromechanical actuators (for example, pistons driven by an electric motor), which exert the braking action on the relative brake discs via the pads. The electronic control unit also receives information from the sensors associated with the braking system regarding the braking action exerted by the electromechanical actuators, in order to accomplish a closed-loop feedback control (for example, via a proportional-integral-derivative controller—PID). In particular, the electronic control unit receives information on the pressure exerted by each actuator on the respective brake disc.
Pressure sensors with a high full-scale value are needed for measuring this pressure. In fact, the force with which the pads are pressed against the disc can have values from 0 up to a maximum in the range 15000-35000 N. The piston acting on the pads has a section of approximately 2 cm2 and hence the pressure sensors must be capable of working up to full-scale values of around 1700 Kg/cm2 or higher (2000 Kg/cm2, for example). Furthermore, the need is felt to perform a pressure measurement with a double measuring scale, i.e., to measure both low pressures with a first precision and high pressures with a second precision, lesser than the first precision.
At present, sensors are known which are capable of measuring high pressure values, these being made with a steel core on which strain gauge elements are fixed. Under the effect of pressure, the steel core deforms according to Hook's Law:ΔL=E·σwhere ΔL indicates the geometric variation of a linear dimension of the core, E is Young's Module of the material constituting the core and σ is the pressure acting on the core in a direction parallel to the deformation dimension. The strain gauge elements detect the geometric deformation of the core to which they are associated via changes in electrical resistance. However, for reasons of reliability, dimensions and costs, these sensors are only applicable to and utilizable for the purposes of characterization and development of a braking system of the previously described type, not the production phase. In addition, they do not have high precision and are equipped with a single measuring scale.
Integrated pressure sensors, made using semiconductor technology, are also known. These sensors typically include a thin membrane suspended above a cavity formed in a silicon body. Piezoresistive elements connected to each other to form a Wheatstone bridge are diffused inside the membrane. When subjected to pressure, the membrane deforms, causing a change in resistance of the piezoresistive elements, and therefore an unbalancing of the Wheatstone bridge. However, these sensors cannot be used for the measurement of high pressures, as they have low full-scale values (in particular, of around 10 Kg/cm2), significantly lower than the pressure values that are generated in the previously described braking systems.